For production of a hydrogen-dominated gasification gas from coal or other solid fuel, a circulating fluidized-bed gasification system has been proposed which comprises, for example, a combustion furnace for burning an unreacted raw material or char with air to heat a bed material, a cyclone collector for receiving a combustion gas from the combustion furnace to collect the bed material mixed in the combustion gas and separate an exhaust gas, a gasification furnace for gasifying a raw material such as coal or biomass into an gasification gas through formation of a fluidized bed by introduction via a downcomer of the bed material collected by the cyclone collector, introduction of the raw material and introduction of water vapor as a gasifying agent and a feed passage for feeding of the bed material and the unreacted raw material or char not gasified in the gasification furnace to the combustion furnace.
The exhaust gas introduced from the combustion furnace into the cyclone collector and separated from the bed material is guided together with a reducing agent such as ammonia or urea to a catalytic denitrator with a denitration catalyst so that nitrogen oxides (NOx) in the exhaust gas are decomposed into and reduced to harmless nitrogen (N2) and water vapor (H2O). Methods of removing the nitrogen oxides (NOx) in the exhaust gas include a method of feeding ammonia or urea as a reducing agent to the catalytic denitrator under a temperature atmosphere of 350 to 400° C. to reduce the nitrogen oxides to nitrogen on the catalyst and a method of spraying ammonia or urea into a temperature atmosphere of the order of 700 to 1000° C. in the combustion furnace to reduce the nitrogen oxides to nitrogen through a non-catalytic gas phase reaction. Even if either denitration method is employed, ammonia or urea needs to be fed as a reducing agent.
On the other hand, the gasification gas produced in the gasification furnace is guided to a tar reformer disposed in a gasification gas line for tar reformation or the like and then is fed to an ammonia remover for removal of ammonia in the gasification gas. It has been known that several percent by weight or more nitrogen is contained in coal fed as a raw material to a gasification furnace and even more nitrogen is contained in the case of biomass, and therefore the gasification gas obtained by gasification of such raw material contains high concentration of ammonia which may bring about problems such as catalyst poisoning in a downstream catalytic process. Therefore, this ammonia is separated and removed by the ammonia remover.
The ammonia remover is such that the gasification gas is contacted with an absorption liquid in the form of, e.g., water in an ammonia absorption column to absorb and remove ammonia from the gasification gas and that the ammonia-absorbing absorption liquid is fed with a stripping gas such as vapor or air in an ammonia stripping column to strip and discharge the ammonia as an ammonia off-gas. The ammonia off-gas, which has a high concentration of ammonia and may bring about the catalyst poisoning problem in the downstream catalytic process, is guided to a treatment device for treatment thereof. Specifically, in the treatment device, the ammonia off-gas is burned in a combustion furnace and, for removal of nitrogen oxides resulting from the combustion, is further processed in a catalytic denitrator similar to the above-mentioned catalytic denitrator for decomposition into and reduction to harmless nitrogen (N2) and water vapor (H2O).
With respect to an ammonia treatment of a coke oven off-gas (COG) in the iron industry, it is described that ammonia separated is burned or recovered as liquid ammonia (Non Patent Literature 1).
In another denitration treatment, ammonia recovered from a cleaning liquid is distributively injected into gas inlet and outlet sides of a regenerated gas combustion furnace. The ammonia is burned together with a sulfuric compound in the combustion furnace for oxidative decomposition; the exhaust gas with the injected ammonia at the gas outlet side of the combustion furnace is guided to a catalytic ammonia reduction denitrator for denitration treatment while kept in high temperature (Patent Literature 1).
Citation List
Non Patent Literature
[Non Patent Literature 1] “AROMATIC AND TAR INDUSTRY HANDBOOK”, 3rd Edition, pp. 66-67, The Japan Aromatic Industry Association, Inc.
Patent Literature
[Patent Literature 1] JP2000-290668A